The invention relates generally to surface acoustic wave (SAW) devices and more particularly to SAW transducers with overlapped electrode fingers.
Surface acoustic wave (SAW) devices are often employed as filters or resonators in high frequency applications. A SAW device contains a substrate of piezoelectric material. For piezoelectric material, a propagating surface wave is accompanied by an electric field localized at the surface, and this enables the wave to be generated by applying a voltage to an array of metal electrodes on the surface. The electrode array is known as an interdigital transducer. Up until this point, prior art methods for tuning SAW devices have concentrated on the weighting of electrodes, as will be further described hereinbelow. However, the methods for determining how to weight the electrodes have heretofore been working on the assumption that SAW velocity dispersion effects can be ignored, and have assumed that SAW velocity value is an independent constant.
U.S. Pat. No. 5,831,492 to Solie discloses transducers which convert input electrical signals to surface acoustic waves propagating upon the surface of the substrate, and then reconvert the acoustic energy to an electric output signal. The input and output transducers are frequently configured as interdigital electrode fingers, which extend from pairs of transducer pads. Alternating electrical potential coupled to the input interdigital transducers induces mechanical stresses in the piezoelectric substrate. The resulting strains propagate away from the input transducer along the surface of the substrate in the form of surface acoustic waves. These propagating surface waves arrive at the output in an interdigital transducer where they are reconverted to electrical signals.
U.S. Pat. No. 5,818,310 to Solie discloses an electrical filter which transmits signals having frequencies within certain designated ranges or passbands, and suppresses signals having other frequencies outside the passband, or within attenuation bands. An ideal filter would transmit the signal within the passband without attenuation and completely suppress signals within the attenuation bands. In practice, known filters do attenuate the passband signal due to absorption, reflection, or radiation, which results in a loss of desired signal power. Further, such filters do not completely suppress signals within the attenuation bands. The use of surface acoustic wave devices as filters or resonators is well known for having the advantages of high Q, low series resistance, small size, and good frequency temperature stability when compared to other frequency control methods such as LC circuits, coaxial delay lines, or metal cavity resonators.
Trivial periodical uniform interdigital transducers have a sin(x)/x passband shape which is not a preferred filter shape because its transition bandwidth is equal to the filter bandwidth, and more importantly, the first sidelobe is typically only 13 dB below the main response. To synthesize arbitrary passbands, transducer weighting is employed. Filtering is thus accomplished in the process of generating the surface acoustic wave by the input transducer, and in the inverse process of detecting the wave by the output transducer. The most effective filtering is preferably accomplished if both input and output transducers are weighted, and thereby participate in the filtering process. Common transducer weighting techniques include apodization and withdrawal weighting. Apodization is typically used for wideband filters and either apodization or withdrawal weighting typically used for narrowband filters.
The term weighting as used herein refers either to the transducers topology or to the separate finger geometry and polarity. Prior art uses the topology weighting mechanisms which are directly associated with amplitude and phase of currents on electrode fingers and of voltages in the gaps between electrode. Such currents and voltages are considered as the sources of the launched SAW or the result of the detected SAW.
Weighting by apodization refers to the varying of the inter electrode overlapping length.
Capacitive weighting refers to a topology implementation, wherein the weights associated with fingers, are mainly defined by overlaps, both by some far fingers as well as by the nearest fingers.
Withdrawal weighting is the isolation or omission of selected fingers which roughly controls currents and voltages.
Series-block weighting is a kind of capacitive weighting.
Line-width weighting refers to an interdigital transducer, having fingers with different widths, and allows a weak weighting of currents and voltages.
Tapering of the transducer, i.e. weighting that leads to fanned topology, is a weighting in the direction along the electrode finger""s length. Such a weighting controls the phase shift of the resonant frequency along the finger""s length, because the period of the finger array changes along that direction.
Phase weighting, i.e. phase modulation that leads to chirp transducer, is achieved by varying the distance between fingers along the direction of SAW wave propagation.
There follows a more in-depth description of certain prior art techniques for weighting electrode fingers. Apodization varies the length of the electrodes to achieve an electrode weighting. With apodization, Fourier transform techniques can be readily applied for computing a filter impulse response when defining a special geometric pattern for the interdigital transducer fingers. It is well known that it is not practical to have an input apodized transducer launching a wave directly into an .output apodized transducer, because an apodized transducer launches a wave which has a non-uniform beam profile, and as a receiving transducer, it must see a uniform beam profile. If a surface wave incident upon an apodized transducer is not uniform over the entire width of the beam, the frequency response changes dramatically. For this reason, apodized input and output transducers can not be used to form a filter unless an added structure such as a multi-strip coupler is used. The multistrip coupler positioned between the apodized input and output transducers, transfers energy from a non-uniform beam into an adjacent track, in which a surface acoustic wave is launched as a uniform beam, and is thus compatible with an apodized transducer receiving the uniform beam. However, using an apodized input transducer for generating a surface acoustic wave, and transmitting the wave through a multistrip coupler to an apodized output transducer, widens the filter device, thus requiring increased space within electronic systems seeking to be ever more miniaturized. Further, apodized transducer-to-apodized transducer through a multistrip coupler is only useful on high coupling substrates such as lithium niobate, whereas, in fact, it is not practical on quartz.
Apodized interdigital transducer structure typically has a lot of small overlaps of the electrode fingers. This leads to diffraction spreading of the partial narrow SAW beams and consequently to additional insertion loss (i.e. diffraction loss) and distortion to the SAW device""s response. The diffraction effect is not easy for mathematical simulation, and requires sophisticated techniques to be accounted in a synthesis problem. As described in the book by D. P. Morgan titled xe2x80x9cSurface-Wave Devices for Signal Processingxe2x80x9d, in order to simplify these difficulties, either the simplified diffraction model called xe2x80x9cparabolic approximationxe2x80x9d might be used for ST-quartz substrate, or the diffraction effect is ignored for the minimal-diffraction orientation Y,Z of the lithium niobate substrate. As described in U.S. Pat. No. 6,031,315 to Abbot, the important criterion in the choice of a piezocrystal cut, is the minimal-diffraction orientation, by way of example, for quartz it reduces degrees of freedom for choosing the cut of either the greatest temperature stability or the most appropriate coupling coefficient. An additional difficulty arises here because the diffraction pattern is found to be very sensitive to the details of the SAW velocity anisotropy.
An unweighted transducer is also used with an apodized transducer if less sidelobe rejection can be tolerated for the system. By way of example, a SAW filter design may require performance that an apodized transducer can provide, yet places constraints on filter size or piezoelectric substrate type such that the use of a multistrip coupler is precluded. An alternative approach often includes the use of withdrawal weighting of one transducer with apodization of the other. A transducer having withdrawal weighting launches a uniform wave across the beam profile, and thus is compatible with an apodized transducer. In withdrawal weighting techniques, electrode fingers are selectively removed, or withdrawn, from a uniform interdigital transducer having constant finger overlap in order to attain a desired transducer response. Since the remaining electrodes all have constant overlap, the withdrawal weighted interdigital transducer can be used with the apodized transducer, amplitude weighted, without the need for the multistrip coupler. Good sidelobe suppression can be obtained using this combination of overlap and withdrawal weighting. It is thus attractive for SAW transducers used on low-coupling piezoelectric substrates such as quartz, where the use of multi-strip couplers is normally impractical. However, since a filter approximation deteriorates if too many electrodes are withdrawn, this technique is limited to narrowband filter applications where the number of interdigital transducer electrodes is large. Further, although the withdrawal weighted transducer satisfies the uniform wave condition, withdrawal weighting is a coarse weighting technique, and as a result produces generally poor xe2x80x9cfar outxe2x80x9d sidelobe response, with less than desirable noise rejection. The approximation of the withdrawal weighted transducer characteristics becomes coarse, if the SAW velocity dispersion effect is ignored for high coupling substrates such as lithium niobate. This occurs because the SAW velocity measured over the interval of isolated electrode fingers, differs from the SAW velocity measured over the interval of charged electrode fingers.
Another way to control the frequency characteristics is by using tapered interdigital transducers. The basic tapered transducer has been reported in the literature and in particular in an article by P. M. Naraine and C. K. Campbell tilted xe2x80x9cWideband Linear Phase SAW Filters Using Apodized Slanted Finger Transducersxe2x80x9d , for Proceedings of IEEE Ultrasonics Symposium, October ""83, pp. 113-116, and article by N. J. Slater and C. K. Campbell titled xe2x80x9cImproved Modeling of Wide-band Linear Phase SAW Filters Using Transducers with Curved Fingersxe2x80x9d, IEEE Transactions on Sonics and Ultrasonics, vol. Su-31, No. 1, January 1984, pp. 46-50. C. K. Campbell, et al., discusses wide band linear phase SAW filters using apodized slanted, or tapered, finger transducers. The frequency performance control is based on the changing of distance between adjacent finger""s centers in the direction of electrode finger""s length and so the interdigital electrode finger array is fitted up to the band pass frequency spectrum. Tapered finger transducer geometries had all the transducer fingers positioned along lines which emanate from a single focal point.
U.S. Pat. No. 4,600,905 to Fredricksen discloses an attempt to improve performance by using the electrode fingers posited into grooves etched into the substrate. The grooves are deeper at the high frequency side of the substrate. The grooves enhance the electromechanical coupling of high frequency components, and thus provide a flatter amplitude response curve. U.S. Pat. Nos. 4,635,008; 4,908,542; 5,831,492 and 5,818,310 to Solie disclose other attempts to improve performance by using hyperbolically tapered electrodes. This improvement is theorized to be based on the fact that there is a hyperbolic dependence between the resonance frequency and the distance between adjacent finger""s centers of the periodical uniform interdigital transducer, and it was assumed that the SAW velocity dispersion effect might be ignored when hyperbolically tapered electrode fingers are used.
There are described three types of weighting that are used individually, and in combination, to provide improved frequency selectivity in U.S. Pat. Nos. 5,831,492 and 5,818,310 to Solie. These are series-block weighting, line-width weighting, and capacitive weighting. Series-block weighting and line-width weighting are therein described for preferred alternate embodiments of that art in the mentioned art U.S. Pat. Nos. 5,831,492 and 5,818,310 to Solie, however, the line-width weighting is useful when combined with series-block weighting. Unlike line-width weighting techniques used in the past, limited because of limited tap weight range, when combined with series-block weighting, the achievable tap weight range is much broader. All these three types of weighting are based on effects of electrostatic charge distribution that depends on the geometry of the electrode fingers, their locations and polarities. The SAW velocity dispersion effects were ignored, and the interdigitized electrode finger""s line-width weighting technique assumes that SAW velocity value is an independent constant. These approximations are acquitted when the interdigitized electrode fingers are implemented from lightweight metal, for example aluminum, and its thickness is rather small, and when the SAW device operating frequency is rather low.
A multi-strip coupler, i.e., a plurality of electrically disconnected interdigitized electrode fingers, is used in many devices in order to couple interdigital transducers whose active regions do not overlap. The multi-strip coupler also is a SAW transducer, and, as described in an article by L. P. Solie titled xe2x80x9cA SAW bandpass filter technique using a fanned multi-strip couplerxe2x80x9d, Appl. Phys. Lett., 30, 374-376 (1977), a bandpass filtering might be achieved using multi-strip couplers. A weighting of such a multi-strip coupler transducer was realized with whole-number quanta of fingers in a section. Several methods of introducing weighting for multi-strip couplers are described in an article by M. Feldmann and J. Henaff titled xe2x80x9cDesign of multi-strip arraysxe2x80x9d, xe2x80x94 IEEE Ultrasonic Symp., 1977,. pp.686-690. In particular, one of them is based on an apodization weighting of an electrode fingers length. In another case, multi-strip coupler transducers electrode fingers in one of tracks are fanned in the same manner as in a regular tapered transducer.
In order to decrease the insertion loss of the SAW filter, SAW Single Phase Unidirectional Transducers (SPUDT) are applied. An effect of the directivity is achieved by using of electrode fingers, which tend to reflect the, originating SAW waves launched or detected within the SPUDT. The reflected SAW waves should cancel the originating SAW waves, which propagate in the unwanted direction, and intensify the originating SAW waves which propagate in the desired direction. SPUDT based single SAW filters are inherently narrow-band devices, as described, for example, in an article by C. B. Saw and C. K. Campbell, titled xe2x80x9cImproved Design of SPUDT For SAW Filtersxe2x80x9d , xe2x80x94IEEE Proceedings at 1987 Ultrasonics Symposium, Denver Colo., November 1987. A withdrawal weighting technique for a group-type SPUDT was presented in an article by D. P. Morgan and P. Durrant, titled xe2x80x9cLow Loss Filters Using Group-Type SPUDTxe2x80x9d, IEEE Ultrason. Symp., 1990,. pp.31-35. Several methods of weighting SPUDT transducers were suggested in U.S. Pat. Nos. 5,831,492 and 5,818,310 to Solie. These methods are based on such techniques as transducer tapering, series-block capacitive weighing and line-width weighting. All of the weighting techniques discussed hereinabove, as applied to SPUDT transducers, use launching/detecting and reflecting fingers, which have either a uniform width or width configured by the tapering of a transducer. Such techniques, which ignore both the SAW velocity dispersion effect and the SAW reflection coefficient dispersion effect, lose accuracy for heavy, thick metal electrode fingers, as well as for high frequencies.
The SAW velocity dispersion effect in interdigital transducers is well known in literature. As described, for example, in an article by D. P. Chen and H. A. Haus titled xe2x80x9cAnalysis of Metal-Strip SAW Gratings and Transducersxe2x80x9d, IEEE-Transactions on Sonics and Ultrasonics, vol. SU-32, No 3, May 1985, in regular interdigitalized electrode finger array the SAW velocity depends on the electrical and mechanical load of the electrode fingers. The SAW velocity depends on the material both of the piezoelectric substrate and the electrode fingers. In the approach of the variational principle, the SAW velocity is expressed with two additive terms. The first term is defined by electrical boundary conditions, and therefore, by the piezoelectrical constants of the piezosubstrate. The second term is defined by the mechanical load, i.e. by densities and elastic constants both of the piezosubstrate and the electrode finger material, as well as by the periodicity of the fingers and their geometry thickness and width. Both of the terms vary smoothly with an electrode finger""s width. Calculations show that if an electrode finger""s width is in the range from 25% to 60 of the distance between the adjacent finger""s centers, both of the terms are approximately proportional to the electrode fingers width. So if the electrode fingers change in width along their length, the SAW velocity dispersion is expected to occur in the same direction.
There is therefore a need in the art for a system to provide an additional weighting mechanism and techniques are needed for fine adjustments of SAW device characteristics at higher frequencies.
Accordingly, it is a principal object of the present invention to overcome the limitations of existing surface acoustic wave (SAW) devices, and to provide improved methods for designing SAW bandpass filters having a SAW transducer with overlapped electrode fingers.
It is a further object of the present invention to provide methods for designing a transducer for a SAW device with an interdigitized series of non-uniform width electrode fingers, thereby increasing the SAW velocity dispersion effect inherent in such a transducer.
it is still a further object of the present invention to provide an additional weighting mechanism for methods and apparatus for a SAW device transducer having an interdigitized series of non-uniform width electrode fingers.
It is yet still a further object of the present invention to provide methods for achieving fine adjustments of characteristics at higher frequencies of a SAW device.
It is yet a further object of the present invention to provide SAW devices having electrode fingers which have been shaped in order to achieve a weighting which is resultant from a change in the SAW velocity dispersion effect along the lengths of the electrode fingers.
It is moreover an object of the present invention to provide SAW devices having electrode ringers which have been shaped in order to achieve a weighting which is resultant from a change in the SAW reflection coefficient dispersion effect along the lengths of the electrode fingers.
These objects, and others not specified hereinabove, are achieved by an exemplary embodiment of the present invention, wherein electrode fingers, which are implemented in form either like a pair of curled brackets or like pair of rounded brackets or like rhombus, are employed for use in surface acoustic wave (SAW) transducers. Three specific features of the curl, round and rhombus shaped electrodes are widening of the frequency band pass, improving the band pass performance, and focusing of the SAW waves. These three features are caused by the SAW velocity dispersion effect. The interdigitalized electrode finger weighting mechanism, based on the SAW velocity dispersion effect, is applied for the SAW device characteristics improvement
The optimization procedure for the high precision design of frequency performance is more effective, since width weighting by means of curled-brackets shaped electrodes adds a degree of freedom. In order to improve attenuation of sidelobe levels around the band pass, in a particular sub-optimal case, the curl""s form is calculated as inverse-cosine. In trivial cases, e.g. where frequency performance precision of the waves is not a primary objective, the curled-brackets shape may either be smoothed out to a rounded-brackets shape or be straightened out to a rhombus shape and yet still achieve a far better SAW device over prior art devices.
Use of a shaped electrode finger, having a width that changes along its length, also causes a weak dispersion of electromechanical coupling coefficient and causes electrostatic charge to be distributed from the top to bottom of that electrode. This is an additional inherent tool with which to form frequency performance of SAW signals.
The curl, round and rhombus shaped electrodes operate like focusing lenses for SAW waves. The electrodes, implemented in the shape of a pair of brackets turned inside out, operate like scattering lenses for SAW waves. Thus, precise control over the diffraction-effect becomes attainable due to the variable width and height of such lenses, and due to a technique whereby each long electrode is implemented in the form of several bracket pairs connected to each other. This also liberates the choice of the piezocrystal cut, that is, a piezoelectric cut with either the greatest temperature stability, or the most appropriate coupling coefficient or a compromise between the two, ignoring or almost ignoring the criterion of minimal-diffraction.
A reflecting electrode finger, that changes in width along its length, changes in its reflection coefficient along that direction. This gives an additional important tool with which to form characteristics of reflected SAW signals.
The SAW velocity dispersion effect for SAW""s propagating under electrode fingers, which change in width along their length, is increased by increasing the thickness of the electrode fingers, and is also increased when a metal heavier than commonly applied aluminum is used as a conductive material for electrode finger fabrication. So the choice of the electrode""s material, and the choice of the thickness, together become the additional degree of freedom for building a weighting function for an interdigital transducer, and for achieving the desired frequency performance.
Prior art weighting mechanisms are based on the distribution of currents and voltages within the SAW transducer. By contrast, weighting to purposely affect the velocity dispersion effect, and other such effects described hereinbelow, is one of the primary teachings of the embodiment of the present invention.
The SAW velocity dispersion depends on both electrical and mechanical load, i.e., it depends on the material both of the piezoelectric substrate and the electrode fingers and depends on the thickness, configuration, polarity and arrangement of the electrode fingers
Prior art generally relied on aluminum, the density and elasticity of which are similar to the substrate""s density and elasticity respectively, thereby substantially avoiding having to consider the velocity dispersion effect. As mentioned hereinabove, the velocity dispersion effect has heretofore been generally perceived as confusing the predictability of results and not something which should be purposely manipulated.
Use of a heavier metal, for example, gold, for fabricating electrode fingers which change in width along their length, increases the SAW velocity dispersion effect, which is a primary objective of the present invention.